Heart valve disease continues to be a significant cause of morbidity and mortality, resulting from a number of ailments including rheumatic fever and birth defects. Currently, the primary treatment of aortic valve disease is valve replacement. Recent statistics show that valvular heart disease is responsible for nearly 20,000 deaths each year in the United States, and is a contributing factor in approximately 42,000 deaths. Worldwide, approximately 300,000 heart valve replacement surgeries are performed annually, and about one-half of these patients received so-called mechanical heart valves, which are composed of rigid, synthetic materials. The remaining patients received bioprosthetic heart valve replacements, which utilize biologically derived tissues for flexible fluid occluding leaflets. In general, bioprosthetic valve replacements have good hemodynamic performance and do not require the anticoagulation therapy necessary for mechanical heart valves. However, these bioprostheses sometimes fail as a result of calcification and mechanical damage.
Flexible leaflets used in heart valves are typically made from bioprosthetic homograft or xenograft materials. For example, the most successful bioprosthetic materials are whole porcine valves and separate leaflets made from bovine pericardium stitched together to form a tri-leaflet valve. In addition, flexible leaflets formed of polymeric, fiber-reinforced, and other synthetic materials have been proposed. The most common bioprosthetic valve construction includes three leaflets mounted around a peripheral support structure with free edges that project toward an outflow direction and meet or coapt in the middle of the flowstream.
Aortic stenosis is abnormal narrowing of the aortic valve. A number of conditions cause disease resulting in narrowing of the aortic valve. When the degree of narrowing becomes significant enough to impede the flow of blood from the left ventricle to the arteries, heart problems develop.
Aortic stenosis is characterized by a significantly higher than normal pressure gradient across the aortic valve. Studies have demonstrated that 85% of patients with surgically untreated aortic stenosis die within 5 years after the onset of symptoms. It follows that an important characteristic of a replacement aortic valve is minimal aortic pressure gradient, especially in symptomatic patients. Many aortic prosthetic valve manufacturers place emphasis on the placement of the prosthesis (sub-annular, intra-annular and supra-annular) in order to draw attention to the importance of implanting a prosthesis with the largest possible effective orifice area. Supra-annular placement (where the sewing cushion lies above the aortic annulus) is often preferred because usually a valve with a larger internal orifice diameter can be implanted. In patients with small aortic roots, either due to anatomy, physical stature, or severe calcification, only the smallest-sized valves (e.g., 19 mm) may be used. Sometimes an even smaller valve would be desirable, but valves smaller than 19 mm are not commercially available. Moreover, even with a supra-annular implant, the smallest prostheses may result in pressure gradients of between 20 and 60 mm Hg and clinically significant aortic stenosis. In all sizes, but in particular with small valves, a reduced gradient that approaches a human native heart valve is preferred and thought to improve long term patient survival rates.
Typical bioprosthetic heart valves have a rigid structure that supports the flexible leaflets. The constant valve seat shape helps ensure reliable closure of the leaflets. Many such artificial valves have circular suture rings made rigid by a metal insert so that they cannot adjust to the natural changes in size of the aorta which surrounds them after the implantation. A common design is a suture ring around a stent structure with three, equiangularly-spaced projecting legs or commissures which extended substantially parallel to one another in the axial direction of the ring. The stent supports three, separate leaflet cusps, and the suture ring interconnects the bottom portions of the projecting legs thereby preventing free, radial movement thereof. To overcome these disadvantages, flexible artificial heart valves have been proposed, such as in U.S. Pat. No. 4,291,420 to Reul, and U.S. Pat. No. 6,558,418 to Carpentier, et al. Though these designs are promising, the conventional rigid circular base remains the dominant surgeon preference.
Bioprosthetic heart valves made by Edwards Lifesciences of Irvine, Calif. have demonstrated excellent durability and hemodynamic performance for a majority of patients without the need for anti-coagulation therapy, which is usually required for mechanical heart valves. In particular, Edwards' bioprostheses such as the PERMOUNT line of valves with pericardial leaflets offer superior hemodynamic performance compared to those with porcine leaflets.
Another issue for some patients with low cardiac output or anticipated recovery difficulty is a small amount of regurgitation associated with some bioprosthetic valves. The three flexible leaflets in a typical bioprosthetic valve meet or coapt within the flow orifice but tend to separate at their convergence in the very middle, which allows a small amount of regurgitation. Though most patients tolerate such minor regurgitation, any increased demand on the heart is problematic for very sick patients and may lead to a slower recovery.
In view of actual and perceived drawbacks associated with current bioprosthetic heart valves, a valve across which there is a minimal pressure gradient and reduced regurgitation is desirable.